I. Field of the Invention
This invention relates to the control of rotary calcining kilns used for calcining various materials, e.g. coke and alumina, and to a control system suitable for the process.
II. Discussion of the Prior Art
Rotary calcining kilns are used in industry to heat or calcine various materials. Such rotary calcining kilns normally consist of a refractory-lined elongated hollow cylinder (often referred to as a "shell"), which can be rotated about its longitudinal axis and which has a feed inlet at one end and a discharge outlet at the opposite end. The tubular shell slopes downwardly from the inlet end to the outlet end by a normally fixed angle from the horizontal, so that material moves gradually along the inside of the shell from the inlet to the outlet by the effect of gravity as the shell is rotated. The interior of the shell is heated by means described more fully below so that the material is heated or calcined as it moves through the interior of the shell.
When certain materials are heat treated in rotary calcining kilns in this way, the nature or yield of the product material may be affected by the operation of the rotary calcining kiln, so that it is desirable to control the rotary calcining kiln in a precise manner particularly suited for the material undergoing the heating process. In particular, it is often desirable to control the temperature profile of the material as it moves through the rotary calcining kiln, but it is usually difficult to measure the temperatures within the rotary calcining kiln with any degree of accuracy because of the harsh conditions which normally exist in the rotary calcining kiln interior, so precise control of the temperature profile is often difficult or impossible.
The calcination of petroleum coke illustrates the difficulties which can be encountered. Petroleum coke is used for the manufacture of carbon electrodes, linings for aluminum reduction cells, and the like. The coke undergoes a calcination step in a rotary calcining kiln in order to remove moisture and combustible volatiles from the green coke and to shrink the coke to a desired density, e.g. up to about 2.1 g/cc. It is also important to bring about an increase the average crystal length (L.sub.c) of the coke from about 15 .ANG., to about 32 .ANG., during the calcination step because products of long average crystal length are more useful and valuable. However, it has been difficult to achieve suitable crystal length enlargement in a consistent manner because the control variables of the rotary calcining kiln seem to affect crystal length in ways which are difficult to predict.
As the coke moves through the rotary calcining kiln, hot combustion gases, produced for example by burning oil or natural gas, may be introduced into the rotary calcining kiln near its outlet end in order to supplement the existing flow of hot combustion gases running counter current to the movement of coke through the rotary calcining kiln. The combustion gases heat the green coke to the desired calcination temperature as it passes through a high temperature calcination zone in the interior of the rotary calcining kiln. In the calcination zone, volatile materials are driven from the coke in such amounts that a fluidized bed is normally established in the mass of coke particles. Tertiary air is often introduced into the rotary calcining kiln in order to enhance the combustion of volatiles, thus forming a so-called "fireball" inside the rotary calcining kiln, and the various resulting gaseous products are discharged from the calcining zone at its upper end. The combustion of the volatiles inside the rotary calcining kiln permits a reduction of the amount of fuel required to bring about the desired calcination process. However, if the calcined coke discharged from the rotary calcining kiln is too hot as a result of the burning of the volatiles within the rotary calcining kiln, the yield of coke product may be decreased because of rapid combustion that takes place as the hot coke encounters the open air.
In U.S. Pat. No. 3,966,560 which issued on Jun. 29, 1976, and is assigned to Alcan Research And Development Limited, a process is disclosed in which the tertiary air is introduced in a controlled manner into a rotary calcining kiln through tuyeres at points spaced from both the green coke inlet and the calcined coke outlet. By carefully controlling the amount of tertiary air and the speed of travel of the coke through the rotary calcining kiln, the hottest part of the coke bed can be kept at a suitable distance from the discharge end of the rotary calcining kiln, thus allowing the calcined coke to cool to some extent before it is discharged into the atmosphere and consequently reducing combustion losses. In this process, nearly all of the heat required for calcining the coke can usually be produced by the combustion of the volatiles driven out of the green coke feed material, and yet a good yield of calcined product can be obtained.
For maximum efficiency of operation of the above type of rotary calcining kiln, the tertiary air should be introduced into the rotary calcining kiln in the region of the maximum evolution of volatiles, i.e. in the region of maximum temperature of the coke. Since the air inlets are fixed, this means that the rate of travel of the coke through the rotary calcining kiln must be adjusted, e.g. by adjusting the rate of rotation of the rotary calcining kiln and/or the amount of green coke fed to the rotary calcining kiln, so that the maximum temperature region coincides with the air inlets. It is also necessary to control the amount of tertiary air introduced into the rotary calcining kiln in such a way that the volatiles are properly burned, but the solid coke material is not consumed. In order to control the operation of the rotary calcining kiln, the temperature of the coke is measured in various regions by means of optical pyrometers and adjustments are made to the speed of rotation, air flow, feed of green coke, etc., to maintain optimal conditions. Additionally, television cameras may be used to observe the position of the fluidized region of the coke and adjustments made to the variables when the fluidized region appears to move away from the desired position in the rotary calcining kiln.
In Canadian Patent No. 1,052,313 issued on Apr. 10, 1979 to Merlyn M. Williams and assigned to Alcan Research And Development Limited, the difficulty of using optical pyrometers to measure temperatures within the rotary calcining kiln is addressed, taking into special consideration the fact that a discharge of smoky gases can lead to a false reading of an optical pyrometer located at the discharge end of the rotary calcining kiln. The solution to this problem, according to this patent, is to provide an additional optical pyrometer spaced from the discharge end of the calciner and to operate an air supply when the output from the additional pyrometer becomes unstable in order to prevent a smoky discharge zone from affecting the output of the additional optical pyrometer.
While this presents a solution to the problem posed by smoky discharge zones, it does not overcome the more basic problem that optical pyrometers are not capable of producing very reliable temperature readings even when smoky conditions are avoided and that their reaction time to temperature variations can be slow. Furthermore, the pyrometers cannot be designed to make precise measurements at locations positioned deeply within the kiln.
In the absence of accurate, fast and reliable temperature readings suitable for most processes, it has been found that it is particularly difficult to operate rotary calcining kilns at maximum efficiency, i.e. at maximum throughput, while reliably producing a properly calcined coke having high L.sub.c values and a high yield. As will be appreciated, at high rates of throughput, departures from ideal temperature conditions within the rotary calcining kiln must be detected and corrected rapidly, otherwise the process quickly becomes unstable and efficiency declines. Moreover, the effects of adjustments of individual variables used for the control of the rotary calcining kiln on the yield and nature of the calcined product can only be understood and predicted if the temperature changes produced by those adjustments can be detected quickly and reliably.
While the above description has been concerned with coke rotary calcining kilns, rapid and reliable control of rotary calcining kilns used for heating or calcining other materials, such as alumina, is also desired in order to improve product quality and yield. For example, the activity of activated alumina is affected by the temperature profile of the alumina as it passes through the rotary calcining kiln.
The keys to achieving proper control of such processes are the accurate, rapid and reliable measurement of temperatures at strategic points and the location of the calcination zone within the rotary calcining kiln. However, such measurements have been difficult or impossible in the past, e.g. by using optical pyrometers and remote television cameras. While the use of thermocouples could conceivably make it possible to measure temperatures more rapidly and reliably, thermocouples have not been used in rotary calcining kilns of this kind because the high temperatures and highly abrasive conditions to which they are inevitably exposed result in an operational lifetime which is so short that the use of exposed thermocouples cast directly into the refractory lining becomes impractical. An additional problem is that there is no procedure to replace the damage thermocouple.
U.S. Pat. No. 4,834,648 to Angelo relates to the treatment of contaminated earth in a rotary calciner to destroy the contaminants. A series of data collection devices is mounted along the heating plenum and these devices are said to be capable of measuring various factors, such as the temperature and gas make-up at all stages of the process. It is also stated that the devices are protected against the extreme heat of the kiln by reason of cooling of the chamber by cooling gas or other gas circulating therein. However, the use of cooling gases to protect temperature sensors is clearly not desirable in most calcination processes because this would affect the desired temperature profile within the kiln. Moreover, it is not clear how Angelo avoids rapid abrasion of the data collection devices, but perhaps this is unnecessary given the type of material being calcined.
Even if accurate and rapid temperature measurements could be made at points within the rotary calcining kiln, however, there are still so many variables that can affect the temperature profile, particularly for coke calcination, that temperature information in itself might not be enough for efficient rotary calcining kiln operation and the production of a desirable product. There is thus a need not only for a way of quickly and accurately measuring temperatures within a rotary calcining kiln, but also a way of using that temperature information to control operational variables, such as the speed of rotation of the rotary calcining kiln, the rate of input of material, the tertiary air flow rate, etc., in order to achieve optimal rotary calcining kiln operation.